1. Field of the Invention
The present invention relates to a method of coating a fluorocarbon resin, a sliding member coated with a fluorocarbon resin using such a method, and a gas compressor having such a resin-coated sliding member.
2. Description of the Related Art
Typically, the respective sliding surfaces of sliding members capable of sliding on each other are required to have wear resistance and seizing resistance. For a vehicle gas compressor of a car air conditioner, for example, significant reduction in weight is often required for its structural components. In this case, therefore, the sliding member may be made of a light-weight material such as an aluminum alloy or the like.
Even though the use of an aluminum alloy or the like achieves the light weight, the surfaces of the sliding members exhibit poor wear resistance and seizing resistance when they slide on each other. Therefore, there may be a case where a fluorocarbon resin such as polytetrafluoroethylene (PTFE) may be coated on the sliding surface of one sliding member. Such a coating avoids poor lubrication, wear, and seizing, which can be caused by the sliding movement between sliding members made of the same metal.
In the case of the fluorocarbon resin coating, there is a need to provide a strong adhesion between the aluminum alloy or the like and the fluorocarbon resin coating layer formed on a base material, for preventing such layer from being detached from the base material by a sliding movement. For improving the adhesion between them, several attempts have been conducted in the prior art. In JP 2000-170657 A (paragraph number 0020, FIG. 2), for example, there is disclosed an invention for making the coating layer itself be easily adhered to an aluminum alloy or the like, in which a coating layer comprised of a fluorocarbon resin and a binder is applied on the surface of a piston made of an aluminum alloy in a swash plate type compressor to attain a strong adhesion between the AI alloy and the coating layer through the binder.
For attaining an improvement in adhesion by providing a coating substrate such as an aluminum alloy with a rough surface, an invention in which this is achieved by etching procedures has been proposed, for example in JP 5-209300 A (Claims) and JP 6-65799 A (paragraph numbers 0014–0018, FIG. 3). In addition, there has been proposed an invention in which this is achieved by means of improvement of coating layer in addition to the etching procedures, for example in JP 5-84468 (paragraph numbers 0008, FIG. 2). Although each of these conventional techniques for improving the adhesion is somewhat effective, its effect is not satisfactory for application to the sliding surface of a sliding member that performs a sliding movement under severe load conditions.
For attaining an improvement in adhesion by providing a coating substrate such as an aluminum alloy with a rough surface, furthermore, there has been proposed an invention in which this is achieved by shot-blasting, for example in JP 2001-263226 A (paragraph numbers 0038–0040).
The fluorocarbon resin coating, which includes a step of surface-roughening with shot-blasting, is generally performed by the process shown in FIG. 15.
In FIG. 15, at first, a base material such as an aluminum alloy is washed in the step of a substrate-treatment (first wash, 801). Then, the washed base material is subjected to shot-blasting using a shot material such as fine particles of hard aluminum oxide of several tens of micrometers in diameter (802) to roughen and clean the surface of the base material. Such a step of surface-roughening and cleaning improves the adhesion between the surface of the base material and the fluorocarbon resin coating and the base material.
Next, the shot material is removed from the surface of the base material (sand shakeout, 803) and is then washed (second wash, 804). Then, the surface of the base material, which has been roughened and cleaned, is subjected to a chemical conversion treatment (805) for preventing the surface from oxidation and so on. Subsequently, the surface of the base material is washed again (third wash, 806) and is then subjected to the steps of fluorocarbon resin coating (807) and baking (808), completing coating of the fluorocarbon resin on the surface of the base material.
The shot-blasting is a process of impinging shot materials upon the surface of the base material, so that minute unevennesses can be formed on that surface. Therefore, the fluorocarbon resin can be introduced into minute concave portions to allow a significant improvement in adhesion property.
The shot material to be used in the above shot-blasting is introduced deep into the minute gaps formed on the base material at the time of performing the shot-blasting (802), so that it often cannot be completely removed by the subsequent steps of sand shakeout (803) and second wash (804). Furthermore, if the base material is a casting product, the shot material, in particular, can easily remain since there are many minute gaps.
Furthermore, the hard shot material remaining in the base material coated with the fluorocarbon resin can be used by being incorporated into a product, for example a gas compressor or the like.
The residual shot material can be left out of the shot material by operation (e.g., a rotation of a rotor or a jet flow of an inner fluid) of the, above product in use. The removed hard shot material is then introduced into a sliding portion or the like of the product, so that a coating layer or a metallic part of the sliding portion or the like may be damaged and eventually baked, resulting in disabled operation.
The conventional method of coating a fluorocarbon resin on a base material made of an aluminum alloy or the like as described above is liable to cause peeling of the coating layer from the base material with an extended period of use. Particularly, such peeling of the coating layer can easily occur when the maximum load is high, the load-change cycle amplitude is large, or the lubricating conditions on startup are poor. For instance, the peeling of the coating layer can easily occur on the sliding surface of a rotor, cylinder, side block, or vane of a vane-type gas compressor.